The present invention relates to an output power control circuit for controlling the output level of a RF (radio frequency) signal amplifier employed in a mobile radio apparatus such as a mobile telephone and, more specifically, to an output power control circuit which supplies output power level control signals to a RF signal amplifier to determine the output power level of the RF signal amplifier selectively.
A mobile radio apparatus must be formed in a compact construction so that the mobile radio apparatus can be placed in a small space in an automobile or so that the mobile radio apparatus is handy for carrying about. Accordingly, the mobile radio apparatus employs compact electronic parts to reduce the size of the casing thereof. Reduction in size of the casing entails a problem that the internal temperature of the mobile radio apparatus, namely, the temperature inside the casing, is caused to rise to an upper limit temperature, for example, 85.degree. C., guaranteeing the rated performance of the mobile radio apparatus in a short time by the heat generated by the electronic components, mainly by the heat generated by an RF signal amplifier.
Particularly, when the RF signal amplifier provides RF signals at the maximum power, for example, three watts, the RF signal amplifier generates heat at a high rate, which further reduces the time in which the internal temperature of the casing rises to a predetermined upper limit temperature.
Incidentally, the output power level of the RF amplifier is maintained at a stable output power level by an automatic power control circuit (hereinafter abbreviated as an "APC" circuit).
As described in detail in U.S. Pat. Nos. 4,523,155 and 4,602,218, the conventional APC circuit maintains the output power level of the RF signal amplifier at one of a plurality of output power levels which are selected by a plurality of level control signals, namely, parallel input bit patterns, and level control signals to be supplied to the APC circuit are set according to power setting command signals transmitted through a control channel or a voice channel by a master station. Accordingly, the mobile radio apparatus is unable to lower the output power level thereof independently even when the internal temperature of the casing thereof reach the predetermined upper limit temperature. In such a case, the mobile radio apparatus disconnects the power source automatically to interrupt the transmission of RF signals, namely, to prevent communication, until the internal temperature drops below the upper limit temperature.